mathowie
Aug 24, 2008
5:16 PM

I think it's totally ok to have seemingly contradictory advice in the tips section since there are many edge-case situations where the opposite of any common tip may be true. I keep an open mind when I'm approving them and generally go for a broad range of tips and might overlap or contradict each other, as long as the message of the tip is a good one.

In this specific case, I think we're talking about two different situations. The jackrabbit starts are referring to the time that you car leaves from a stoplight, and generally I'm saying going from 0mph-10mph by stomping on the gas is a horrible idea and it wastes a lot of gas (though leaving off the line very very slowly still gets poor but not as bad as jackrabbit start mileage for reasons covered in the second tip).

The second tip is referring to the behavior of internal combustion engines, which generally do poorly at fuel efficiency below about 40mph and generally peak around 50-55. There are times that if you are going 20mph and you're entering a freeway, it's fine to stomp on the gas and just get the 20-55mph acceleration over with quickly so you can get the car into a more efficient state and start saving fuel.

I got a ScanGauge the other day and have been monitoring my mpg closely in real time and I'm planning to write it up here in the Fuel Talk forum because it reveals the truth to a lot of things like that second tip.

@mathowie

"I got a ScanGauge the other day and have been monitoring my mpg closely in real time and I'm planning to write it up here in the Fuel Talk forum because it reveals the truth to a lot of things like that second tip."

I'd like to see the results.

I think one of the big issues with the fuel tips is that strategies vary quite a bit between cars. Let's say you drive an American sedan with a pushrod V6. In this situation, the engine is generally optimized for lower RPM, and you'll get better fuel economy keeping the revs below 2500-3000 while accelerating, since this rev range is where the engine produces power most efficiently. On an economy car with a DOHC 4cylinder, you'll do better by revving a bit higher while accelerating, since the most efficient operation on these engines is most like the 3000-5000 rpm range.

I do take serious issue with the one tip titled "Watch your RPMs." While I agree that high revs do reduce fuel economy by increasing engine friction, the posters begins by claiming "Despite common belief speed is not really connected to MPG." This is in fact completely untrue. Drag forces dominate at highway speeds. The drag force acting on your vehicle is equal to the drag coefficient * frontal area of the vehicle * square of the velocity of the vehicle. Therefore, every time you increase your velocity by ~1.4, you double the amount of power you're using to overcome drag, e.g. drag forces are twice as high at 79 mph as they are at 55 mph. These forces quickly overcome engine / drivetrain losses.

mathowie
Aug 25, 2008
10:36 AM

well rem83, with my scangauge installed, I can see what the "Watch your RPM" tip refers to. There are specific low rpm points where my automatic transmission shifts up and if you can hold the speed and rpm down you can get a pretty significant mpg boost (on my truck going 44mph is enough to get into the top gear and on flat ground my ~18mpg truck reports 25-35mpg at that speed/rpm)

I don't have a problem with the tip itself, the part I have the problem with is where the tipster completely discounts the effects of aerodynamic drag and claims that it's a common misconception.

@ rem83

I assume you are talking about my tip "Watch you RPM's" as I don't see where the two quoted tips mention aerodynamics.

In any case, I was referring to velocity as a determining factor in MPG as a misconception. I never meant to imply aerodynamics did not have an effect.

rem83 please read your quote carefully. It says exactly the opposite you seem to think it says. He's saying that a lot of people seem to think speed (and therefore drag) doesn't have an effect on MPG but he himself believes it does.

@nakio30 - The way I interpreted the quote included a comma between the words belief and speed. If you interpret it to say that "speed is not really connected to MPG" is the common belief, then it is poorly worded and a sentence fragment (it would be better to say "despite the common belief that speed is not connected to mpg" which would still be a fragment), which seemed less likely to me than a missing comma, particularly given the emphasis of the tip on engine RPM and not total speed.

That and Tiki's previous comment both seem to suggest that your interpretation is actually incorrect and the sentence should read "Despite common belief, speed is not really connected to mpg."

Also thought I'd throw this article from tirerack.com out there: http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=29

While it's certainly not perfect, the author's estimate is that at highway speeds aerodynamic losses account for about 60% of your vehicle's fuel consumption, while driveline friction (essentially engine RPM - of course, the article doesn't take into account efficiency changes over the engine's available RPM) is about 15%. During city driving, driveline friction accounts for ~45%

sbilik
Sep 9, 2008
11:23 PM

@mathowie After a few fillups and calibrations, you'll find your new ScanGauge gives a more accurate results. But even if it's initially off by a few percentage points, the relative feedback is still useful for fuel usage feedback.

Here's My take on this whole issue. I have owned and driven many cars and trucks, both with Manual Transmissions and Automatics. If you are in control, ie manual shifting, you can get exceptional mileage. I averaged 23-25 combined mpg in a 2000 Corvette with the manual. I accomplished this by accelerating in 1st, 2nd briskly, but not fast just enough to stay away the 1-4 skip shift, then letting the engine rpm drop until I could get into 6th and lope at 40-45. I also used this technique in my 2003 C2500hd Duramax, a got 19-21mpg around Phoenix. The Blazer that I have listed currently has an 700-r4 automatic, I accelerate smoothly up to at least 45, so it can shift into 4 and lockup the torque-converter, if I don't get above 42 or so it will hang in third gear at 1750 rpm, but in 4th around 1270 rpm or so.
That's My .02
Later

I forgot to finish my thought. Engines are more efficient operating under a load, Higher numerical gears. Keeping an engine rpm high and lightly loaded wastes fuel, a medium loaded engine in it's operating range will be more efficient. Their is no hard a fast rule that you can only shift into 5th after you get to 50mph, if your engine has enough torque, up-shift, it will save you fuel. One of the reasons I dislike Automatics, another is they take some HP to turn their oil pumps, and are generally heaver than their manual counterparts.
Later